Molecular sieves are a commercially important class of materials that have distinct crystal structures with defined pore structures that are shown by distinct X-ray diffraction (XRD) patterns. The crystal structure defines cavities and channels/pores that are characteristic of the specific type of molecular sieve: this is usually described as the framework type or topological type. A full listing of framework types is maintained by the IZA (International Zeolite Association) http://www.iza-structure.org/databases/. Such framework types or topological types are not defined by composition only by the arrangement of the T-atoms (tetrahedral atoms) that bound the channels/pores and cavities and make up a structure. Thus a framework type or topological type is unique and is provided with a unique three letter code by the IZA.
An example of a topological type (framework type) is CHA. The CHA topological type (framework type) is designated after the natural mineral chabazite. Various synthetic versions of the CHA topological type have been described most notably SSZ-13 (Zones, S. I. U.S. Pat. No. 4,544,538, (1985)) that has a composition in terms of silica:alumina ratio distinct from the natural mineral and SAPO-34 Lok, B. M., Messina, C. A., Patton, R. L., Gajek, R. T., Cannan, T. R. and Flanigen, E. M. J. Am. Chem. Soc., 106, 6092-6093 (1984)) that has a framework composition consisting of SiO2, AlO2 and PO2 tetrahedra (i.e. it is not an aluminsosilicate but a silicoaluminophosphate). Chabazite (the natural mineral), SSZ-13 and SAPO-34 all have the same framework structure (topological type) but have different compositions. Such differences in composition have a profound influence on the properties, hence usefulness in terms of industrial applications, of a material.
Molecular sieves have numerous industrial applications, and zeolites of certain frameworks, such as CHA, are known to be effective catalyst for treating combustion exhaust gas in industrial applications including internal combustion engines, gas turbines, coal-fired power plants, and the like. In one example, nitrogen oxides (NOx) in the exhaust gas may be controlled through a so-called selective catalytic reduction (SCR) process whereby NOx compounds in the exhaust gas are contacted with a reducing agent in the presence of a zeolite catalyst. In another example, molecular sieve zeolites having the CHA framework type have found application in the conversion of methanol to olefins (MTO) catalysis. In the case of MTO catalysis it is the silicoaluminophosphate composition of the CHA topological type that has found utility in MTO catalysis.
As has been noted, two types of molecular sieves are zeolites and SAPOs. Zeolites were traditionally considered to be crystalline or quasi-crystalline aluminosilicates constructed of repeating TO4 tetrahedral units with T bring most commonly Si and Al (although other T-atoms such B, Fe and Ga have been described). These units are linked together to form frameworks having regular intra-crystalline cavities and/or channels of molecular dimensions. Numerous types of synthetic zeolites have been synthesized and each has a unique framework based on the specific arrangement its tetrahedral units. By convention, each topological type is assigned a unique three-letter code (e.g., “GME”) by the International Zeolite Association (IZA). SAPO's have a three-dimensional microporous crystalline framework of PO2+, AlO2−, and SiO2 tetrahedral units. Because an aluminophosphate (AlPO4) framework is inherently neutral, the incorporation of silicon into the AlPO4 framework by substitution generates a net negative charge that, similar to aluminosilicate zeolites, can be used to generate acid sites. Controlling the quantity and location of silicon atoms incorporated into an AlPO4 framework is important in determining the catalytic properties of a particular SAPO molecular sieve.
The catalytic properties of a SAPO catalyst also can be modified after the SAPO molecular sieve has been synthesized. This type of “post-synthesis” modification is accomplished by treating the molecular sieve with metallic, semi-metallic or non-metallic materials comprising nickel, cobalt, manganese, magnesium, barium, strontium, lanthanides, actinides, fluorine, chlorine, chelating agents, and others. The modifiers may or may not become part of the final composition of the modified catalyst.
The majority of the framework types described by the IZA require an organic template or structure directing agent (SDA) to facilitate their synthesis. With very few exceptions the SDA is incorporated into the molecular sieve framework during the synthesis process and has to be removed, usually by a heat treatment stage, before the channels/pores and cavities can be “freed up” and made accessible to other molecules.
As an example, synthetic zeolites of the SFW topological type when prepared as aluminosilicate compositions are produced using structure directing agents (SDAs), also referred to as a “templates” or “templating agents”. The SDAs that are used in the preparation of aluminosilicate SFW topological type materials are typically complex organic molecules which guide or direct the molecular shape and pattern of the zeolite's framework. Generally, the SDA can be considered as a mold around which the zeolite crystals form: hence the early descriptor of “template”. After the crystals are formed, the SDA is removed from the interior structure of the crystals, leaving a molecularly porous aluminosilicate cage.
In typical synthesis techniques, solid zeolite crystals precipitate from a reaction mixture which contains the framework reactants (e.g., a source of silica and a source of alumina), a source of hydroxide ions (e.g., NaOH), and an SDA. Such synthesis techniques can take several hours to days (depending on factors such as crystallization temperature and the specific zeolite) to achieve the desired crystallization. When crystallization is complete, the solid precipitate containing the zeolite crystals is separated from the mother liquor which is discarded. This discarded mother liquor frequently contains unused SDA, and this may be wholly or partially degraded due to harsh reaction conditions, and unreacted silica.
The SFW framework has been recently discovered and synthesised as an aluminosilicate (SSZ-52) by Zou and coworkers (D. Xie, L. B. McCusker, C. Baerlocher, S. I. Zones, W. Wan, X. Zou, J. Am. Chem. Soc., 2013, 135, 10519-10524) The use of a polycyclic quaternary ammonium cation (N,N-diethyl-5,8-dimethyl-2-azonium bicyclo[3.2.2]nonane) as SDA leads to the formation of a stacking-faulted structure.
There is a need to develop new zeolites having the basic structure of known zeolites, where minor changes in the structure can affect one or more of the catalytic properties of the zeolite. In some cases, while minor changes in the structure may not be discernable using normally used analytical techniques, the catalytic activity of the structurally modified zeolite may be improved relative to very closely related analogous zeolites. Unexpected improvements in the catalytic activity of such structurally modified zeolites can allow for the compositions of exhaust gases from engines to meet various regulatory requirements.